1. Field
This invention relates to a multiplexer for wireless communication and a wireless receiver.
2. Related Art
When a certain frequency band is allocated to a wireless carrier, the allocated band is often divided into plural divisional channels (frequency bands). In a conventional base station receiving system, the divisional channels that collectively received are frequency-converted to baseband (BB) respectively and thereafter processed separately for the respective channels.
When each of the divided and converted signals passes an amplifier and a mixer, each signal generates intermodulation distortion due to non-linearity of the amplifier or the mixer. Now, the intermodulation distortion refers to the frequency component of 2f1−f2 and 2f2−f1 generated for two fundamental waves with frequencies of f1 and f2. Namely, the signal in a certain band generates the intermodulation distortion within the pertinent band itself and also generates the intermodulation distortion with equal upper and lower bandwidths outside the band.
Thus, in the intermodulation distortion in a certain channel range, the intermodulation distortion due to the upper and lower channels thereof is added to the intermodulation distortion generated by the pertinent channel itself. When the signal power in an intermediate channel is small, the distortion for the signal, i.e. noise with respect to the signal increases. This deteriorates equivalent carrier to noise ratio (equivalent C/N) and causes the wireless communication unstable.
IEEE Microwave Theory and Techniques vol. 44, 1213 (1996) or IEEE Microwave Theory and Techniques Symposium Digest 295 (1997) discloses a multiplexer including a small number of Chebyshev filter using a superconductive material. These multiplexers cannot branch channels for communication into each channel.
Electronic Letters 29, 1674 (1993) or IEEE Microwave Theory and Techniques Symposium Digest 1604 (2001) discloses a multiplexer for branching adjacent channels. The multiplexer includes a waveguide. However, a waveguide for branching a low frequency range becomes large.
If the respective channels are separated at an RF stage by the multiplexer, the intermodulation distortion includes only by the channel itself. This gives a merit of improving the equivalent C/N. However, in order to separate the respective channels, the multiplexer has to include band-pass filters for very small bandwidths. Further, in order to assure sufficient attenuation in adjacent channel ranges, the filters have to have a very steep skirt characteristic.
A multiplexer for separating adjacent channels has been known. However, this multiplexer includes a band-pass filter for a certain-channel, which has an attenuation pole in a band for an adjacent channel. Thus, a communication signal is attenuated transiently. In short, conventionally, the multiplexer capable of separating the respective channels for communication could not be realized and so a wireless receiver with the respective channels separated at the RF stage could not be realized.